This invention relates to the testing of printed circuit boards and, more particularly, to the sensing of excessively thin strip conductors by use of burnout current pulses.
Printed circuit boards are manufactured in large numbers for the electronics industry. Such boards are often constructed with many strip conductors interconnecting numerous electronic components. Some of the strip conductors may be lengthy as compared to dimensions of the circuit board. The generally accepted mode of manufacture employs photolithography for the production of a laminated circuit board wherein an outer metallic layer is etched to form thin ribbonshaped conductor strips disposed on an insulating substrate.
The current carrying capacity of a strip conductor is dependent on the cross sectional area of the conductor and, hence, on the thickness and the width of the strip conductor. The thickness of a strip conductor is fixed by the thickness of the metal layer from which the conductor is formed. The width of the strip conductor is established by the photolithography process, and may be selected during the design of masks used in the photolithography process to impart a desired current-carrying capacity to the strip conductor.
A problem arises in that the width of a strip conductor cannot be controlled as precisely as is desired in the manufacture of a printed circuit board. As a result, some variations occur in the width characterized by both increases and decreases from a nominal or preselected value of the width. Of particular concern is a reduction in width, or necking down of the conductor, which presents a hazard in that the circuit board may fail by either of two modes.
The conductor may break because of physical stresses which build up in the region of reduced cross section. Such breakage inhibits the flow of current in the conductor with an attendant failure in operation of an electric circuit on the circuit board. Alternatively, the conductor may present a high resistance to the flow of electric current in the region of reduced cross section. The resultant flow of current may be too small for proper operation of the electric circuit. Also, a flow of electric current through the region of reduced cross section may produce excessive heating, or hot spot, which may eventually melt the conductor leaving an open circuit which stops further flow of the current.
One attempted solution to the foregoing problem involves visual inspection of the circuit board to ascertain whether all of the strip conductors are properly formed. However, the visual inspection has not been wholly satisfactory in that it is dependent on human judgment, the necked down region may be irregularly shaped, and there is no accurate measurement of the cross section.
Visual inspection can be facilitated by a burnout technique wherein a pulse of current is applied to the strip conductor, the magnitude of the current being large enough to melt of vaporize the reduced cross-sectional area leaving a visible opening in the conductor. However, such current pulsing must be carefully controlled so as to avoid damaging other portions of the circuit board. The technique had been avoided with particularly delicate circuit boards because presently available current regulation circuits do not provide as high a level of control as would be desirable. Also, circuitry for the detection of a burnout, such as by differentiating a current waveform, does not provide as high a level of control as would be desirable. Even with the aid of the burnout technique, the visual inspection is time consuming and subject to human error and, hence, disadvantageous.